This invention is addressed to a method of making a diffusion bonded composite structure consisting of metal selected from the group consisting of titanium and titanium-based alloys and comprising a relatively fragile cellular core component bonded between face sheet components, one of which includes an expanded internal cooling channel produced by superplastic forming through the use of an inert gas supply to displace one wall surface of the face sheet component into machined recesses in the cellular core component where the displaced wall surface is diffusion bonded to the cellular core component.
Actively-cooled panel structures have been proposed in the past for use with various articles of manufacture including aircraft and particularly supersonic aircraft. Recent technological concepts include the proposal for a fuel-cooled honeycomb panel. Apart from the concept of an actively-cooled panel, a titanium honeycomb panel with evacuated cells will provide more effective insulation than a similar aluminum structure. Moreover, aerospace structures made from titanium or titanium alloys have a greater strength to weight ratio as compared to similar structures made from aluminum. In addition, titanium aerospace structures will withstand higher temperatures than aluminum structures.
In my prior U.S. Pat. No. 4,013,210, assigned to the Assignee of this invention, there is disclosed a method of producing a composite structure consisting of a honeycomb panel joined to other structure by diffusion bonding to form a load-carrying member. The component parts of a composite structure are urged together under a moderate pressure transmitted through a glass pad to assure a uniform contact force while carrying out the diffusion bonding process. Disclosed in U.S. Pat. No. 3,633,267 issued to the same Assignee as this invention in the name of Czeslaw Deminet et al., is a method of diffusion bonding a metallic honeycomb structure to face sheets. A honeycomb core is placed in a furnace with a face sheet held above the honeycomb core by means of heatyieldable spacers. When the temperature in the furnace is elevated to a diffusion bonding temperature, e.g., 1700.degree. F., the heat-yieldable spacers deform and the upper face sheet descends into contact with the honeycomb core. A compressive force is applied through a heat-yieldable glass pad to insure that all portions of the face sheet are urged under uniform pressure into proper diffusion bonding contact with the honeycomb core.
Active-cooling by fuel-cooled passageways in a composite honeycomb structure offers many advantages particularly at selected locations in hypersonic aircraft. A fuel-cooled panel structure is particularly useful at sites where, during a mission, temperatures are encountered which are higher than acceptable even when the more effective insulation is provided by evacuated cells in a titanium honeycomb structure. The present invention is, therefore, addressed to providing a titanium honeycomb structure with evacuated cells together with passageways for a coolant medium in a face sheet of the honeycomb structure. A major advancement in the art by the present invention is active cooling of the face sheet of the panel to thereby prevent overheating of the skin due to aerodynamic friction. The technique of superplastic forming of titanium or titanium alloys is utilized as part of the present invention to form the ducting within a face sheet of the composite structure. The concept of superplastic forming is per se known in the art and discussed in recent literature include an article by J. W. Edington, entitled "Physical Metallurgy of Superplastic Metals Technology", Metals Technology, March 1976, pages 138-153. Superplastic forming is also discussed in a paper by F. H. Froes et al entitled "Microstructural Control in Titanium Alloys for Superplastic Behavior" presented at the "Forging and Properties of Aerospace Materials Conference", Leeds, England, Jan. 5-7, 1977. Superplastic forming is a process founded by the extreme ductility of certain metals when deformation is carried out at a temperature above about one-half the melting point of the metal. The forces required to deform titanium or known present-day titanium alloys by superplastic forming are relatively small, e.g., as low as 100 psi. In the superplastic forming process, the metal undergoing deformation behaves in a manner similar to hot glass or thermoplastic polymers when deformed.